Liquid crystal displays (LCDs) are passive flat panel displays which depend upon external sources of light for illumination. They are manufactured as segmented displays or in one of two basic configurations. The substrate needs (other than being transparent and capable of withstanding the chemical conditions to which it is exposed during display processing) of the two matrix types vary. The first type is intrinsic matrix addressed, relying upon the threshold properties of the liquid crystal material. The second is extrinsic matrix or active matrix (AM) addressed, in which an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. In both cases, two sheets of glass form the structure of the display. The separation between the two sheets is the critical gap dimension, of the order of 5-10 .mu.m.
Intrinsically addressed LCDs are fabricated using metal deposition techniques, typically at temperatures .ltoreq.350.degree. C., followed by standard metal etching procedures. As a result, the substrate requirements therefor are often the same as those for segmented displays. Soda-lime-silica glass with a barrier layer has proven to be adequate for most needs. A high performance version of intrinsically addressed LCDs, the super twisted nematic (STN) type, has an added requirement of extremely precise flatness for the purpose of holding the gap dimensions uniform. Because of that requirement, soda-lime-silica glass made using the float glass manufacturing process must be polished. Such polishing processes are expensive and time consuming, and generate a large amount of glass particles which have the potential to negatively impact further processing of the glass sheets. Alternatively, glass can be formed using a process which does not require polishing, e.g. fusion downdraw.
Extrinsically addressed LCD's can be further subdivided depending upon the nature of the electrical switch located at each optical element (subpixel). Two of most popular types of extrinsically (or active matrix, AMLCD) addressed LCD's are those based on either amorphous (a-Si) or polycrystalline (poly-Si) silicon thin film transistors (TFT's).
U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) lists four desirable properties for a glass to exhibit in order to fully satisfy the needs of a substrate for extrinsically addressed LCD's:
First, the glass must be essentially free of intentionally added alkali metal oxide to avoid the possibility of alkali metal contamination of the TFT;
Second, the glass substrate must be sufficiently chemically durable to withstand the reagents used during the manufacture of the TFT;
Third, the expansion mismatch between the glass and the silicon present in the TFT array must be maintained at a relatively low level even as processing temperatures for the substrates increase; and
Fourth, the glass must be capable of being produced in high quality thin sheet form at low cost; that is, it must not require extensive grinding and polishing to secure the necessary surface finish.
That last requirement is most difficult to achieve inasmuch as it demands a sheet glass production process capable of producing essentially finished glass sheet. A process capable of meeting this requirement is a particular downdraw process known as the overflow downdraw, or fusion, sheet manufacturing process. The overflow downdraw process is described, for example, in U.S. Pat. No. 3,338,696 (Dockerty) and U.S. Pat. No. 3,682,609 (Dockerty). Fusion formed glass sheets, unlike float glass sheets, are sufficiently flat that they do not need to be polished after forming. Two glasses which meet the above requirements, Corning Incorporated Codes 7059 and 1737 sheet glass, are currently employed as substrates for extrinsically addressed LCD's. These glasses are made using the overflow downdraw process, and hence do not require polishing after forming.
Recent improvements in the resolution of extrinsically addressed LCD's have led to the development of a fifth glass requirement, that is, a high glass strain point. This property is used as an indication of the thermal shrinkage of the glass. As can be appreciated, the lower the strain point, the greater is this thermal shrinkage. Low thermal shrinkage is desirable for precise alignment during successive photolithographic and other patterning steps during the TFT processing. Consequently, glasses having higher strain points are generally preferred for extrinsically addressed LCD's, particularly those which employ poly-Si TFT technology. Thus, there has been considerable research to develop glasses demonstrating high strain points so that thermal shrinkage is minimized during device processing. Corning Code 1737 glass, which has the highest strain point (666.degree. C.) in the AMLCD substrate industry, is rapidly becoming an industry standard. Concurrent with their high strain points, these glasses often have high melting temperatures, e.g. on the order of 1550-1650.degree. C.
Another technology termed "chip-on-glass" (COG) has further emphasized the need for the substrate glass to closely match silicon in thermal expansion. Thus, the initial LCD devices did not have their driver chips mounted on the substrate glass. Instead, the silicon chips were mounted remotely and were connected to the LCD substrate circuitry with compliant or flexible wiring. As LCD device technology improved and as the devices became larger and required finer resolutions, these flexible mountings became unacceptable, both because of cost and of uncertain reliability. This situation led to Tape Automatic Bonding (TAB) of the silicon chips. In that process the silicon chips and electrical connections to the chips were mounted on a carrier tape, that subassembly was mounted directly on the LCD substrate, and thereafter the connection to the LCD circuitry was completed. TAB decreased cost while improving reliability and increasing the permitted density of the conductors to a pitch of approximately 200 .mu.m--all significant factors. COG, however, provides further improvement over TAB with respect to those three factors. Hence, as the size and quality requirements of LCD devices increase, COG is demanded for those devices dependent upon the use of integrated circuit silicon chips. For that reason, the substrate glass preferably demonstrate a linear coefficient of thermal expansion closely matching that of silicon; i.e., a linear coefficient of thermal expansion (0.degree.-300.degree. C.) between about 32-46.times.10.sup.-7 /.degree. C., most preferably 32-40.times.10.sup.-7 /.degree. C.
Many of the glasses manufactured for flat panel display applications, particularly those which are formed by downdraw processes (e.g., the fusion or slot draw processes), are melted or formed using manufacturing equipment comprised of refractory metals, e.g. platinum or platinum alloys, particularly in the fining and conditioning sections of the process, where refractory metals are employed in order to minimize the creation of compositional in homogenieties and gaseous inclusions caused by contact of the glass with oxide refractory materials. In addition, many of these manufacturing processes employ arsenic as a fining agent. This is because arsenic is among the highest temperature fining agents known, meaning that, when added to the molten glass bath, it allows for O.sub.2 release from the glass melt even at high melting temperatures (e.g. above 1450.degree. C.). This high temperature O.sub.2 release, (which aids in the removal of gases during the melting and fining stages of glass production), coupled with a strong tendency for O.sub.2 absorption at lower conditioning temperatures, (which aids in the collapse of any residual gaseous inclusions in the glass), results in a glass product essentially free of gaseous inclusions. In addition, the oxidizing nature of the arsenic fining package allows for protection of the platinum based metal systems by preventing contamination as a result of tramp metals reduction. Other fining agents typically melt and release their oxygen far too early when added as fining agents to high melting temperature glasses and reabsorb O.sub.2 too late during the conditioning process, thereby disabling their fining abilities. From an environmental point of view, it would be desirable to find alternative methods of making such high melting point and strain point glasses without having to employ arsenic as a fining agent. It would be particularly desirable to find methods for making such glasses via downdraw (especially fusion-like) processes. Unfortunately, previous efforts at doing so have been hindered by the production of unacceptable amounts of bubbles in the glass. This has been a particular problem with glasses which employ refractory metals such as platinum or platinum containing alloys in their molten glass delivery systems. This is because platinum can cause an electrochemical reaction to occur with the glass which results in bubble formation on or near the platinum (commonly referred to as blistering) contacting region of the glass.